Cleanup rustdoc::formats::cache::Impl

[camelid]

r? @jyn514

[camelid]

Actually, this is trickier than I thought because it looks like there are parts that need some fields on Item. Also, I know that you (Joshua) are planning to get rid of clean anyway, so perhaps not worth the effort.

[jyn514]

I do think this is a useful change, but I think we should try to implement #83183 first since otherwise we'd end up rewriting a lot of the same code:

error[E0599]: no method named `stable_since` found for struct `types::Impl` in the current scope
    --> src/librustdoc/html/render/print_item.rs:613:43
     |
613  |                     implementor.impl_item.stable_since(cx.tcx()).as_deref(),
     |                                           ^^^^^^^^^^^^ method not found in `types::Impl`
     | 
    ::: src/librustdoc/clean/types.rs:2034:1
     |
2034 | crate struct Impl {
     | ----------------- method `stable_since` not found for this

error[E0599]: no method named `const_stable_since` found for struct `types::Impl` in the current scope
    --> src/librustdoc/html/render/print_item.rs:614:43
     |
614  |                     implementor.impl_item.const_stable_since(cx.tcx()).as_deref(),
     |                                           ^^^^^^^^^^^^^^^^^^ method not found in `types::Impl`